Transparent, substantially solvent-free coatings of the epoxide type have been developed which cure by polymerization of the resinous components, curing being initiated by radiation sensitive initiators which form Lewis acid catalysts upon being subjected to electromagnetic radiation. These coatings avoid many of the problems of conventional protective coatings which are applied from various solvent systems, such problems including the necessity for solvent removal and recovery, the fire and explosion hazards associated with solvent systems, pinholeing of the coating caused by solvent release, softening and running of the coating under the effects of the heat which is required to hasten the process of solvent removal and finally, the possibility of damage to the coating components by the temperatures often encountered in solvent removal.
Although it has been recognized that these and other disadvantages of solvent based coating systems may be avoided through use of the radiation-cured solventless coatings, their use has hitherto been limited to the application of either transparent coatings or lightly pigmented, i.e. up to about 15%, because the presence of opacifying pigments in the coating compositions has seriously impaired or prevented the necessary transmission of the radiation which is required to activate the catalyst precursors causing the curing of the coating by polymerization of the epoxide components. Furthermore, radiation cured coatings containing a pigment in sufficient concentration to effectively mask the substrate (generally, at least about 25-30% titanium dioxide is the minimum pigment concentration necessary for satisfactory masking of the substrate) are generally accompanied by inadequate adhesion to the substrate and/or poor flexibility, inadequate gloss, off-color or lack of whiteness and a tendency to blush or have a chalky appearance after immersion in hot water.
Coatings which are to be applied to a substrate and then overprinted with decorative or identifying indicia must satisfy a number of functional criteria. For example, a radiation-curable coating which is to be applied as an undercoating for the decorative material on a metallic food or beverage container such as a sealed metal can must be capable of smooth, even application in a thin coating, must cure rapidly after exposure to irradiation, must have sufficient opacity to mask the substrate completely, must adhere tightly to the substrate throughout the processing steps of filling and sealing the can, must be sufficiently flexible to withstand forming and sealing operations carried out on the can and the can ends, must be receptive to the printed decorative material to be overlaid thereon and must withstand, without deterioration of any of these properties and without change in the color of the coating, high temperature aqueous medium used in pasteurization of various foods and beverages often packaged in metal cans. These and other criteria are satisfied by the radiation cured, opaque, epoxide type coatings of this invention containing up to about 40% to about 50% of a particular surface treated titanium dioxide pigment which allows rapid cure of the epoxide vehicle components under the influence of electromagnetic or electron beam radiation.
It is therefore an object of the present invention to provide a substantially solventless, pigmented, liquid coating composition of the epoxide type which, upon application to a solid substrate and exposure to suitable radiation, will cure to a tightly adherent white, opaque protective coating which masks the substrate and which is receptive to printing or other decorative or identifying indicia.
It is a further object of this invention to provide epoxide coatings for metal, paper and plastic articles, which coatings contain pigment loadings as high as 40 to 50% by weight of the coating solids and which may be rapidly cured to an opaque protective film after exposure to irradiation.
It is a further object of this invention to provide a substantialy solvent-free, radiation-curable epoxide coating composition containing a high loading of a pigment which does not substantially retard the curing rate of the coating upon exposure to irradiation which imparts a high degree of opacity to the coating.
Further objects will become apparent from the detailed description and examples set forth in the following specification and claims.